1. Field of the Invention
The present invention relates to an apparatus for monitoring the state of a battery, particularly of a storage battery which is in the form of what is called a battery pack formed by assembling a plurality of battery modules each consisting of a plurality of sealed-type nickel-metal hydride batteries or the like, and to be installed particularly in a movable body such as an electric vehicle.
2. Description of the Prior Art
A sealed-type nickel-metal hydride battery is excellent in fundamental properties such as an energy density, an output density and a life cycle and is under development for practical use as a power source of a movable body such as an electric vehicle. When such a battery is to be used in an electric vehicle, a battery capacity and a total voltage are required to be 50 to 120 Ah and about 100 to 350 V, respectively, in order to obtain a predetermined output. In a nickel-metal hydride battery, since an output voltage of one cell which is the minimum unit in a practical use is about 1.2 V, a number of cells are to be connected in series to one another so as to obtain a required total voltage. For example, when 10 cells are connected in series to constitute 1 module and 24 modules are connected in series, a battery pack of 240 cells is formed and a total voltage of 288 V is obtained.
In an electric vehicle using the above-mentioned battery as the power source, a state of ability of the battery must be monitored to always ensure a constant running ability.
Conventionally, a process of monitoring the state of ability of the battery pack is mainly conducted by monitoring the terminal voltage, and specifically, a voltage across the positive and negative terminals of the whole of the battery pack, i.e., the total voltage is directly measured by an instrument.
In the conventional measuring method as described above, the voltage to be measured is high (for example, 288 V), and the high voltage is directly supplied to the instrument. In an electric vehicle, a battery is usually disposed under the floor (outside the vehicle) and therefore there is little fear that a person accidentally touches a high voltage portion of the battery during a normal operation or the like. However, a monitor for displaying the voltage is disposed, for example, in the vicinity of the dashboard inside the vehicle, and therefore a high voltage wire must be led to the dashboard. This requires countermeasures for maintaining insulation to be taken at various portions, thereby producing a problem that a considerably bulky configuration is required, and furthermore causes a serious problem that there is a risk of electrical shock against the human body.
Even when one of modules or cells constituting a battery pack involves a failure (including run-down), such a failure does not largely affect the total voltage. Therefore, a failure of a battery is hardly detected. Moreover, there is a further problem that, even when an existence of a faulty battery is detected, it is difficult to identify a faulty module.